|Publication number||US6919983 B2|
|Application number||US 10/985,514|
|Publication date||Jul 19, 2005|
|Filing date||Nov 10, 2004|
|Priority date||Sep 24, 2002|
|Also published as||CN1492569A, CN100542001C, EP1403210A2, EP1403210A3, US6844960, US20040058469, US20050094243|
|Publication number||10985514, 985514, US 6919983 B2, US 6919983B2, US-B2-6919983, US6919983 B2, US6919983B2|
|Inventors||Marek W. Kowarz|
|Original Assignee||Eastman Kodak Company|
|Export Citation||BiBTeX, EndNote, RefMan|
|Patent Citations (19), Non-Patent Citations (2), Referenced by (1), Classifications (17), Legal Events (5)|
|External Links: USPTO, USPTO Assignment, Espacenet|
This is a continuation of U.S. application Ser. No. 10/253,747, filed on 24 Sep. 2002 now U.S. Pat. No. 6,844,960 by Marek W. Kowarz, and entitled “Microelectromechanical Device with Continuously Variable Displacement”, now U.S. Pat. No. 6,844,960, wherein this patent is incorporated by reference herein.
The present invention relates to electrostatic microelectromechanical devices, and more particularly to microelectromechanical devices with movable members requiring continuously variable and stable displacement over a large travel range.
Many different types of microelectromechanical (MEMS) devices, such as variable capacitors, electromechanical gratings and mirrors, inkjet printheads, and a variety of sensors, rely on electrostatic forces between two electrodes to produce controlled actuation of a movable member. However, as is well known, continuous control of the displacement of the movable member is only possible over a fraction (approximately ⅓) of the distance between the two electrodes because of the nonlinear nature of the electrostatic forces. Once the displacement exceeds this fraction, “pull-in” or “Pull-down” occurs, whereby the nonlinear electrostatic force completely overwhelms the mechanical restoring force of the member.
Different approaches have been used to produce continuously variable displacement in electrostatic MEMS devices while avoiding the pull-down instability. The most straightforward is to design the device with a large enough separation between the two electrodes, thereby enabling sufficient displacement before reaching the instability point. This approach has been used by Silicon Light Machines in their analog Grating Light Valve (GLV), as described by Bloom et al. in U.S. Pat. No. 6,215,579, entitled Method and Apparatus for Modulating an Incident Light Beam for Forming a Two-Dimensional Image, issued Apr. 10, 2001. To avoid high operating voltages caused by increased electrode separation, these analog GLVs are specifically designed to have low mechanical restoring forces. Alternatively, a more complex structural design can be used in an electromechanical grating to obtain continuous actuation over a larger travel range, as described by Hung et al. in U.S. Pat. No. 6,329,738, entitled Precision Electrostatic Actuation And Positioning, issued Dec. 11, 2001 and in E. S. Hung and S. D. Senturia, “Extending the Travel Range of Analog-Tuned Electrostatic Actuators,” Journal of Microelectromechanical Systems, vol. 8, No. 4, pgs. 497-505 (1999). Another alternative is described in U.S. Pat. No. 6,362,018, entitled Method for Fabricating MEMS Variable Capacitor with Stabilized Electrostatic Drive, by Xu et al., issued Mar. 26, 2002, whereby a fixed series capacitor is added to a variable MEMS capacitor in order to extend the electromechanical tunability of the variable capacitor. A disadvantage of this last approach is that the required actuation voltage is raised significantly.
Recently, an electromechanical conformal grating device, an optical MEMS device consisting of ribbon elements suspended above a substrate by a periodic sequence of intermediate supports was disclosed by Kowarz in U.S. Pat. No. 6,307,663, entitled Spatial Light Modulator With Conformal Grating Device, issued Oct. 23, 2001. The electromechanical conformal grating device is operated by electrostatic actuation, which causes the ribbon elements to conform around the support substructure, thereby producing a grating. The device of '663 has more recently become known as the conformal GEMS device, with GEMS standing for grating electromechanical system. The conformal GEMS device provides high-speed light modulation with high contrast, good efficiency and digital operation. However, for applications that require amplitude modulation of light intensity, analog operation with continuous control of the displacement of the ribbon elements is needed. In addition, the approaches mentioned earlier for producing continuously variable displacement while avoiding the pull-down instability are ill-suited for the conformal GEMS device.
There is a need, therefore, for an electrostatic microelectromechanical device that has a continuously variable displacement and avoids the problems noted above.
The above need is met according to the present invention by providing an electrostatic micromechanical device with continuously variable displacement, that includes: a movable member having a first electrode; an opposing surface having a second electrode; a channel separating the movable member from the opposing surface; a liquid situated in the channel, wherein the liquid has a sufficiently high dielectric constant so as to enable continuously variable and stable control of a displacement of the movable member over a travel range spanning at least half of the channel; the displacement being a result of a voltage applied between the first electrode and the second electrode; and at least one solid dielectric layer physically situated between the first electrode and the second electrode.
In its broadest embodiment, the present invention increases the usable travel range of a variety of movable members used in electrostatic microelectromechanical (MEMS) devices. Within the usable travel range, the displacement of the movable members is continuously variable and stable. Although the invention is described primarily for the specific case of an electrostatic MEMS device with one or more tensile ribbon elements, it will be apparent to those skilled in the art that the invention also pertains to devices containing other microelectromechanical structures. These movable members can include, for example, singly-clamped beams, doubly-clamped beams, membranes or rigid plates on torsional hinges. Furthermore, multiple movable members can be used to create more complex moving structures, such as electrostatic comb drives or electromechanical gratings.
In the present invention, the channel 4 is filled with a liquid that has a high dielectric constant and can withstand high electrostatic fields. By careful selection of this liquid, the pull-down instability can be eliminated, thereby enabling continuously variable and stable displacement of the tensile ribbon element 2 a over a travel range spanning the entire channel 4.
The dielectric constant requirements for the liquid can be established by considering the effective electrostatic thickness t of the device. Including the effect of the liquid-filled channel 4, the effective electrostatic thickness t of the layers between the first electrode 6 and the conducting substrate 9 is given by the expression
t=d c +∈t ∈,
is the total dielectric thickness of the solid dielectrics; dc is the depth of the channel 4; and ∈ is the dielectric constant of the liquid. In the total dielectric thickness tm the summation is over all solid dielectrics between the two electrodes with the thickness tm of each solid dielectric reduced by its dielectric constant ∈m. It can be shown that the tensile ribbon element 2 a can be displaced smoothly throughout a travel range spanning the entire channel 4 if the depth of the channel dc is less than approximately 0.388t. To satisfy this requirement, the dielectric constant of the liquid should satisfy the inequality
∈>1.58d c /t ∈ (Equation 1)
A liquid that satisfies Equation 1 eliminates the pull-down instability, thereby allowing for a continuously variable and stable displacement over the entire depth of the channel 4. Liquids with lower dielectric constants can be used to increase the travel range beyond what is usually possible with a gas-filled or evacuated channel 4. Such lower dielectric constant liquids are still considered to be within the scope of this invention.
For the liquid-filled device (∈=18), the midpoint displacement is a smooth function of applied voltage as shown in
As is well known, tensile ribbon elements, such as those discussed above, are basic building blocks that can be used to form more complex MEMS devices. For example, large numbers of parallel ribbon elements can be used to produce electromechanical gratings, such as the conformal GEMS device or the GLV. Individual ribbon elements can be used to make variable capacitors or capacitive sensors.
The conformal Grating Electromechanical System (GEMS) devices disclosed in '663 are illustrated in
A top view of a four-device linear array of conformal GEMS devices 5 a, 5 b, 5 c and 5 d is shown in FIG. 7. The elongated ribbon elements 23 a, 23 b, 23 c, and 23 d are depicted partially removed over the portion of the diagram below the line A—A in order to show the underlying structure. For best optical performance and maximum contrast, the intermediate supports 27 should preferably be completely hidden below the elongated ribbon elements 23 a, 23 b, 23 c, and 23 d. Therefore, when viewed from the top, the intermediate supports 27 should not be visible in the gaps 28 between the conformal GEMS devices 5 a-5 d. Here, each of the conformal GEMS devices 5 a-5 d has three intermediate supports 27 with four equal-width channels 25. The center-to-center separation A of the intermediate supports 27 defines the period of the conformal GEMS devices in the actuated state. The elongated ribbon elements 23 a-23 d are mechanically and electrically isolated from one another, allowing independent operation of the four conformal GEMS devices 5 a-5 d. The bottom conductive layer 12 of
A robust implementation of the invention requires careful selection of both the applied voltage waveform and the liquid. As disclosed by Kowarz et al. in U.S. Pat. No. 6,144,481, entitled Method and System for Actuating Electro-Mechanical Ribbon Elements in Accordance to a Data Stream, issued Nov. 7, 2000, a bipolar voltage waveform reduces charging in electromechanical ribbons. Furthermore, as described by Gooray et al. in U.S. Pat. No. 6,419,335, Electronic Drive Systems and Methods, issued Jul. 16, 2002, a high-frequency bipolar waveform used in conjunction with a liquid-filled electrostatic MEMS device reduces electrochemical reactions and dielectric breakdown of the liquid. To obtain the results shown in
The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the invention.
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|1||Elmer S. Hung and Stephen D. Senturia, "Extending the Travel Range of Analog-Tuned Electrostatic Actuators" Journal Of Microelectromechanical Systems, vol. 8, No. 4, Dec. 1999.|
|2||Michael S.-C. Lu, et al., "Closed-Loop Control of a Parallel-Plate Microactuator Beyond The Pull-in Limit" Solid-State Sensor, Actuator and Microsystems Workshop, Jun. 2-6, 2002, pp. 255-258.|
|Citing Patent||Filing date||Publication date||Applicant||Title|
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|U.S. Classification||359/290, 359/573, 359/295, 385/37, 349/158, 359/566, 359/291, 345/84, 359/231, 359/228, 359/224.1|
|International Classification||G02B26/08, B81B3/00|
|Cooperative Classification||G02B26/0808, B81B3/0037|
|European Classification||G02B26/08D, B81B3/00K2|
|Dec 19, 2008||FPAY||Fee payment|
Year of fee payment: 4
|Feb 21, 2012||AS||Assignment|
Effective date: 20120215
Free format text: SECURITY INTEREST;ASSIGNORS:EASTMAN KODAK COMPANY;PAKON, INC.;REEL/FRAME:028201/0420
Owner name: CITICORP NORTH AMERICA, INC., AS AGENT, NEW YORK
|Mar 4, 2013||REMI||Maintenance fee reminder mailed|
|Jul 19, 2013||LAPS||Lapse for failure to pay maintenance fees|
|Sep 10, 2013||FP||Expired due to failure to pay maintenance fee|
Effective date: 20130719